Retrofittable T-ray antireflection coatings

Abstract

Terahertz time-domain spectroscopy (THz-TDS) is able to extract optical or dielectric properties of materials, whether in the solid, liquid, or gas phase, in the T-ray frequency region. Spectroscopy of a liquid or gas often requires a receptacle to confine the sample. In order to allow T-rays to probe the sample effectively, the receptacle must have T-ray transparent windows. However, even though windows are transparent to T-rays, attenuation exists, because of multiple reflections at air-window and window-air interfaces, which accounts for a major energy loss. Due to the recent emergence of T-ray technology, there has been very little work carried out to-date on the reduction of reflection losses. This paper analyses the reduction of T-ray reflection loss by means of an antireflection coating. Because T-ray wavelengths are much larger than visible wavelengths, the antireflection layer thickness for T-rays is much larger than the usual optical case. This creates an interesting opportunity for retrofittable antireflection layers in T-ray systems. In the experiment, a coating material made from polyethylene sheets is applied onto the surfaces of a silicon window. The coated window shows enhancement of the transmittance within a range of frequencies.

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“…Furthermore, the THz transmittance of several materials, such as ZnSe crystal, alumina ceramic plate, quartz crystal, fused silica, and paper, was measured with the THz radiation generated from this laser system of which the frequencies are 1.07, 1.17, 1.98 and 3.33 THz, unlike the THz time domain spectroscopy (TDS) [7][8][9][10]. Based on above results, competition for gain for ammonia THz radiation with 91 μm and 152 μm pumped by the 9R(14) line was discussed.…”

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“…where n is the refractive index of the material. Two-side reflectivity R ignoring absorption increases to [10]…”

“…The refractive index of SiO 2 is 2.1 in THz range [8,9] , and that of GaAs is near 3.6 [10,11] . As mentioned above, SiO 2 is transparent to THz radiation, so optically polished SiO 2 bears 23.6% reflectivity (two-side).…”

“…As mentioned above, SiO 2 is transparent to THz radiation, so optically polished SiO 2 bears 23.6% reflectivity (two-side). Though GaAs is semitransparent to THz radiation in short wavelengths [10,11] , its reflectivity is still more than 32% (single-side). In conclusion, both windows show relatively high reflection and low absorption for THz radiation.…”